In general, to separate molecular ions to analyze components according to the masses of the ions in a mass analyzer, first, a process of ionizing gaseous molecules is required.
A method of bombarding the gaseous molecules with an electron beam to produce the molecular ions is most frequently used. To produce the electron beam, a device for heating a filament at a high temperature to induce thermionic emission is most widely used.
The filament can be heated at a high temperature by causing a high current to flow to a high-temperature metal such as tungsten or rhenium. However, due to high power consumption, battery power is rapidly consumed in a portable mass analyzer, and a reaction to electron emission caused by a rise to a high temperature is slow. As such, it is difficult to control the electron emission in a mass analyzer that is suitable to produce a continuous output electron beam and requires pulse ionization within a short time.